Electric powered and hybrid powered vehicles are becoming more common as alternatives to vehicles powered solely by internal combustion engines. Hybrid vehicles rely on a combination of energy sources for power, such as the combination of an electric motor and internal combustion engine, or even a fuel cell system. Both electric and hybrid vehicles employ electric drive motors that are powered by an on-board, high voltage DC power supply. This power supply includes a rechargeable battery pack formed of plurality of battery modules connected in a closed series circuit, wherein each of the modules is formed from plurality of individual cells. A voltage detection system is connected with the battery pack to monitor battery voltage, and thus the amount of reserve power present in the battery pack.
The battery pack, for example, can produce 300 volts or more, consequently, for safety and other reasons, it is mounted on the vehicle in a manner that electrically isolates it from both the passenger compartment and from the vehicle's chassis. The ungrounded battery pack is therefore part of an “electrically floating” circuit.
Due to any of a number of possible causes, parts of the battery pack may become electrically coupled with the chassis, creating a low resistance path that allows a leakage current to flow from the battery pack to the chassis. Such leakage currents produce a drain on the battery pack.
Systems have been devised in the past for detecting leakage currents in high voltage power supplies of the type mentioned above. These prior systems employ either an AC detection method or a DC detection method. The AC method involves applying an AC signal to a part of the vehicle's body that is insulated from direct currents by a condenser and a transformer. The DC method measures an electrical leakage resistance as a direct current without the need for a transformer or condenser. The known DC method and its shortcomings can be better understood by referring to FIG. 1 of the accompanying drawings which depicts a leakage detection circuit coupled with a battery pack for powering an electrical traction motor. In FIG. 1, the following conventions apply:
VpTraction battery pack voltageTB+Traction battery pack positiveTB−Traction battery pack negativeLPLeakage pointV1TB+ to leakage point voltageV2Leakage point to TB− voltageRLLeakage point to chassis leakage resistanceRcΣ of measurement circuit resistance except A/Dmeasurement resistorRa2dA/D measurement resistorRΣ of Rc and Ra2d
For simplicity of illustration, the battery pack is represented as comprising only two battery modules, respectively designed by the DC voltage sources V1 and V2. The method is performed by conducting two successive measurements in two separate circuits, respectively. The first of these measurements is performed on a circuit positioned on the low side of the leakage point LP, formed by V1, Rc, switch SW+, Ra2d, and RL. The second of the measurements is performed on a circuit positioned on the high side of the leakage point LP, formed by V2, Rc, switch SW−, Ra2d, and RL.
To perform the first set of measurements, SW+ is closed, I1 and Vp are measured. Then,V1=I1×(Rc+Ra2d+R1)=I1×(R+R1)  (1)
Next, switch SW− is closed, and I2 is measured.
Then,V2=I2×(Rc+Ra2d+RLL)=I2×(R+RL)  (2)∵ Vp=V1+V2  (3)and                                           I            1                    =                                    V              a2d1                                      R              a2d                                      ;                              I            2                    =                                    V              a2d2                                      R              a2d                                                          (        4        )                                          ∴                      R            L                          =                                                            V                p                            ×                              R                a2d                                                                    V                a2d1                            +                              V                a2d2                                              -          R                                    (        5        )            
This prior method is based on the assumption that the any changes in the battery module voltages V1 and V2 are negligible between the points in time that the two sets of leakage detection measurements are taken. As a practical matter, however the voltages V1 and V2 may change during the time between the two measurements, in which case the leakage measurements will be in error. The potential magnitude of this error, and the dependence of the leakage resistance RL on the battery pack voltage Vp, may be better appreciated by performing the following sensitivity analysis using equations (4) and (5):                                           ⅆ                          R              1                                            ⅆ                          V              p                                      =                                            R              a2d                                                      V                a2d1                            +                              V                a2d2                                              =                      1                                          I                1                            +                              I                2                                                                        (        6        )            I1 and I2 are typically designed to meet a standard insulation resistance specification requirement of 500Ω/V (per ISO/DIS 6469-1) in case of SW+ or SW− failing in a short circuit mode. Accordingly, 0≦I1+I24≦mA, and                               ∴                                    ⅆ                              R                1                                                    ⅆ                              V                                  p                  ⁢                                                                          ⁢                  min                                                                    =                  250          ⁢                                          ⁢                      Ω            /            V                                              (        7        )            When the leakage resistance is high, I1+I2 is low, and       ⅆ          R      L            ⅆ          V      p      is high. From the foregoing analysis, it may be appreciated that RL is very sensitive to the changes in Vp. Therefore, it can be seen that when the amount of change in the battery pack voltage Vp is more than a negligible amount during the time the two leakage detection measurements are taken, the calculation of RL from equation (5) is not accurate.
Accordingly, there is a clear need in the art for a method of detecting leakage currents using DC measurement techniques that provides accurate leakage current measurements in spite of fluctuations in battery voltage. The present invention is directed toward satisfying this need.